In the modern society of today, the use of light emitting diodes emitting visible light is wide spread. Applications of light emitting diodes include light emitting signs, such as exit signs or emergency signs, lights for vehicles, such as breaking lights, backlight illumination for large scale video displays and illumination systems for furniture, such as shelves. However, the use of light emitting diodes for applications like projection sources or car headlamps is growing less rapidly, due to the fact that the emission angle of a generic light emitting diode is wide. For these applications, it is desired to use a light source having highly collimated light.
Many light emitting diodes (LEDs) suffer from poor extraction efficiency, i.e. the amount of light escaping from a light emitting diode (LED) compared to the light generated in the LED is small. A common problem of LEDs is that total internal reflection of the light in the LED material causes the light to be trapped within the LED and, hence, it does not escape from the LED. One cause of total internal reflection is that there is a large difference in index of refraction between the light emitting material and air. The light being trapped within the LED will eventually be absorbed and lost. In order to reduce the amount of light trapped due to total internal reflection, a method for increasing extraction efficiency has been proposed. The method achieves improved extraction efficiency by providing roughened areas at the light emitting surface of the LED. Yet further improvement of the extraction efficiency has been achieved by means of providing periodic structures in the LED material. An example of a periodic structure is a photonic crystal (PC). By arranging the photonic crystal close to the active region of the LED, an improvement of the extraction efficiency is obtained. The photonic crystal forms gaps (or cavities), which diffract the light at a certain angle. For purposes of theoretical modeling, the light emission (radiation field) of an LED is divided in a near field, which comprises the electric field near the active layer and the photonic crystal structure and a far field, which corresponds to the actual light emission observed (further away) from the LED. A problem of many light emitting diodes, having a photonic crystal structure, is that the far field emission of the LED displays a regular pattern of brighter and/or darker spots, dots or alike.
In US-patent application 2005/0173714 A1, there is disclosed an epitaxial structure of a solid state lighting system. The solid state lighting system comprises an active layer emitting light in response to current injected into the layer, a first structure adjacent to the active layer, wherein the structure and the active layer is arranged to trap the light generated by the active layer and guide the light in parallel to the active layer, and a second structure arranged on top of the guiding layer intended for extracting the light that is trapped by the first structure, wherein the second structure comprises a plurality of photonic crystal arrays with different parameters. Furthermore, there are many photonic crystals cells in each lighting system. According to one embodiment, the geometrical shapes of photonic crystal cells and electrodes in a chip, are arranged in the form of square cells. Each cells has an electrode, which is non-transparent, and comprising a photonic crystal for extraction of light. An object of the solid state lighting system, disclosed in 2005/0173714 A1, is to provide improved extraction efficiency.
In a solid state lighting system of this type, the photonic crystal cells are repeated over the surface of the lighting system. A disadvantage with such a solid state lighting system is that a combined far field pattern, emanating from a plurality of photonic crystal cells, may be non-uniform, i.e. the far field pattern may have bright spots, dots, circles or alike.